(a) Field of the Invention
This invention relates generally to the art of amperometric measurement and amperometric measuring devices of the type used for quantitative electrochemical analysis methods where the concentration of an electroactive species such as oxygen dissolved in or admixed with a fluid is to be measured or monitored; more particularly, this invention relates to an improved method of thermally protecting an amperometric cell or probe which is temporarily subjected to temperatures outside of the operative cell temperature range, such as in heat sterilization.
The invention further comprises an improved probe provided for passage of a fluid, such as a cooling liquid, through said probe.
(b) Description of the Prior Art
Electrochemical cells of the type used for quantitative electrochemical analysis are well known in the art and generally include a working or sensing electrode having a defined or electroanalytically effective surface, a counter electrode, an optional guard electrode, an electrolyte in contact with the electrodes and a membrane that is substantially impermeable to the electrolyte but is permeable to the electroactive species of interest and defines the sensor face in terms of a barrier between the electrolyte space, notably the electrolyte film on top of the sensing electrode, and the ambient medium that contains the electroactive species.
For amperometric analytical operation, the working electrode of such a cell arrangement is polarized by a constant DC voltage to furnish a current whose steady state magnitude is proportional to the activity of the electroactive species of interest. Cells of this type and their operation and uses are discussed in detail in the following illustrative U.S. Pat. Nos. 2,913,386, 3,071,530, 3,223,608, 3,227,643, 3,372,103, 3,406,109, 3,429,796, 3,515,658, 3,622,488 and 4,096,047 as well as in British Published Application No. 2,013,895.
Structural and operational data of such prior art cells can be found in the literature, particularly in the Monography by Fatt, Irving, "Polarographic Oxygen Sensors", CRC-Press, Inc., USA, 1976, incorporated herein by reference.
The first mentioned U.S. Pat. No. 2,913,386 to Leland E. Clark considered as the pioneering patent in this art already teaches a semi-permeable membrane that restrains the electrolyte and the terms "membrane-covered" or "membrane-enclosed" are being used generally to refer to such electroanalytical devices, e.g. as "membrane-covered polarographic detectors".
As the term "polarography" has also been used for techniques based on the dropping mercury electrode and operating either in a voltametric or galvanic mode, the term "membrane-enclosed amperometric cell" or MEAC is used herein to refer to electroanalytical probes such as the "Clark Cell" and modifications thereof including those that use guard electrodes and various devices to improve operation, reliability, sensitivity and maintenance.
Because of the high sensitivity, e.g. for routine determinations of dissolved oxygen in water in the ppm to ppb range, MEAC-type oxygen probes are of interest in various types of industrial microbiological processes including enzymatic or fermentative methods, where reliable monitoring of the oxygen content is of paramount interest.
Microbiological methods require that the equipment which is exposed to a biologically active and, hence, biodegradable medium must be sterilizable; heat sterilization, such as exposure to temperatures above 100.degree. C., e.g. pressurized steam of 120.degree. C., for periods in the range of minutes or hours is a generally preferred method for heat sterilization of processing equipment, such as fermenters, feeding pipes and other devices that have come, or will come, in contact with a process stream containing biologically active matter.
As a consequence, oxygen probes for use in such processes should be heat-sterilizable but because MEACs include temperature sensitive constituents, one of which is the electrolyte, this presents substantial problems. Thus, when a MEAC typically designed for operation at a temperature between 0.degree. C. and 40.degree. C. is exposed to 120.degree. C. steam, the following damaging effects may occur:
expansion of the volume of the e1ectrolyte relative to the space available, creating internal pressures which tend to stretch the membrane; PA0 shrinkage, softening and creeping of structural parts of the probe producing permanent dimensional changes; PA0 stresses of differential expansion of disparate materials, leading to permanent damage of structural parts; PA0 recrystallization or phase changes of the membrane material causing permeability changes which, even if but temporary, can cause measurement errors for a time; PA0 enhanced solvent power and corrosiveness of electrolyte at elevated temperatures causing attack on structural parts.
A conventional way around these problems is to use a MEAC that can be taken out of the system to be monitored when the latter is heat-sterilized and to effect sterilization by other than thermal treatment. As actual measurement is neither required nor normally possible during heat-sterilization periods, this is feasible per se but requires a substantial procedural effort, aside from the problems of a reliable sterilizing treatment for the probe that does not introduce uncontrolled sterilant residues into the system which is to be monitored.
Cooling of the MEAC would seem to provide another way to resolve heat-sterilization problems; yet, probe cooling implies that exposed parts of the probe do not reach sterilization conditions and remain contaminated.
Finally, use of a non-aqueous electrolyte solvent having a boiling point above 120.degree. C. and a low vapor pressure at that temperature would seem to offer another way around some of the difficulties set forth above but as the requirements for such a solvent are quite demanding, no practicable solution appears feasible.
The operative temperature range of a MEAC containing an aqueous electrolyte, i.e. the temperatures of an ambient medium which do not damage the MEAC or modify its sensitivity, may be broadened somewhat by electrolyte additives, such as thickeners, and such additives are in fact being used in prior art heat-sterilizable sensors which, in addition to electrolyte additives, provide for mechanically compensating the thermal expansion of the electrolyte during in-line heat-sterilization. The disadvantages of such a system are decreased sensitivity and less reliability, prolonged stabilization periods, frequent exchange of membrane and electrolyte, and relatively complicated structures and maintenance requirements.